Insight into the SBU condensation in Mg coordination and supramolecular frameworks: A combined experimental and theoretical study

Article abstract of DOI:10.1021/ja210564a

This work is to emphasize the influence of the synthetic procedures in the isolation of different coordination polymers that coexist under hydro-/solvothermal conditions. An experimental and theoretical study in the Mg²⁺:4,4'-(hexafluoroisopropylidene)bis(benzoic acid):1,10- phenantroline system has been carried out. Computational studies have determined the relative energies for those compounds that coexist under certain hydrothermal conditions, and have helped to identify the driving forces for the formation of the different phases. The five new compounds belong to five different structural types: AEPF-14, which presents two polymorphs (α- and β-) ([Mg(H₂O)₄(phen)₂]L), AEPF-15 ([Mg(HL)₂(phen)]) and AEPF-16 ([Mg(H₂O) ₂(L)(phen)]) are both 1D MOFs (AEPF-16 with a helical structure), and AEPF-17 ([Mg(H₂O)(L)(phen)]) with a 2D structure. Hydrogen bond interactions found in the five compounds have been taken into account to study the topology of their supramolecular nets. Finally, dehydration studies performed on AEPF-14 (α- and β-) and AEPF-16 have shown that the topological type of their supramolecular networks determines the structural changes that take place during the dehydration processes of these Mg compounds.

Full text of DOI:10.1021/ja210564a

Article
pubs.acs.org/JACS
Insight into the SBU Condensation in Mg Coordination and
Supramolecular Frameworks: A Combined Experimental and
Theoretical Study
Ana E. Platero-Prats,† Víctor A. de la Pena-O’Shea,‡ Davide M. Proserpio,§ Natalia Snejko,†
̃
,†
,†
́
Enrique Gutierrez-Puebla,* and Angeles Monge*
́
†Department of New Architectures in Materials Chemistry, Instituto de Ciencia de Materiales de Madrid, ICMM-CSIC,
c/Sor Juana Ines de la Cruz, 3, 28049 Madrid, Spain
‡Thermochemical Process Group, Instituto Madrileno de Estudios Avanzados en Energía (IMDEA Energía),
́
̃
́ ́
Avenida Ramon de la Sagra n° 3 28935 Mostoles, Spain
§Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universita
20133 Milano, Italy
̀
degli studi di Milano, via G. Venezian 21,
S
* Supporting Information
ABSTRACT: This work is to emphasize the influence of the
synthetic procedures in the isolation of different coordination
polymers that coexist under hydro-/solvothermal conditions.
An experimental and theoretical study in the Mg2+:4,4′-
(hexafluoroisopropylidene)bis(benzoic acid):1,10-phenantro-
line system has been carried out. Computational studies have
determined the relative energies for those compounds that
coexist under certain hydrothermal conditions, and have
helped to identify the driving forces for the formation of the
different phases. The five new compounds belong to five
different structural types: AEPF-14, which presents two polymorphs (α- and β-) ([Mg(H2O)4(phen)2]L), AEPF-15
([Mg(HL)2(phen)]) and AEPF-16 ([Mg(H2O)2(L)(phen)]) are both 1D MOFs (AEPF-16 with a helical structure), and
AEPF-17 ([Mg(H2O)(L)(phen)]) with a 2D structure. Hydrogen bond interactions found in the five compounds have been
taken into account to study the topology of their supramolecular nets. Finally, dehydration studies performed on AEPF-14 (α-
and β-) and AEPF-16 have shown that the topological type of their supramolecular networks determines the structural changes
that take place during the dehydration processes of these Mg compounds.
or polymorphism phenomena.4 The insertion of ancillary
ligands in the synthesis media is usually an approach to control
the framework connectivity, and in this case could minimize the
Mg solvation.
INTRODUCTION
■
The current environmental needs, which are becoming more and
more considered in chemistry research, have posed new scientific
challenges. In this sense, obtaining new structural types by using
alkaline-earth elements (AE) could represent a comparatively
cheap, nontoxic, and green alternative to conventional MOFs,
which are mostly based on transition metals or, more recently, rare-
earth elements. However, despite these advantages, the synthesis of
AE-MOFs still remains much less explored, mainly due to the
inherent difficulties arising from these elements such as their
formation/crystallization processes, their unpredictable coordina-
tion numbers and geometries, and the tendency of these elements
to form solvated species.1 The recent efforts made in such a way
have given rise to different exciting novel AE-MOFs with good
performances in gas/liquid separations2 or heterogeneous
catalysis,3 proving the viability of this green alternative.
Continuing our research on AE-MOFs,2b,5 here we report
the synthesis, crystal structures, and characterization of five
novel Alkaline-Earth Polymeric Frameworks (AEPF) based on
magnesium, (hexafluoroisopropylidene)bis(benzoic acid) (H2L,
from now on), and 1,10-phenantroline (phen, from now on)
ligands. These five AEPFs exhibit 0D, 1D, and 2D
dimensionalities, and 2D and 3D supramolecular frameworks:
a molecular magnesium material with polymorphism (named
AEPF-14, α- and β-phases, [Mg(H2O)4(phen)]L), a 1D MOF
(AEPF-15, [Mg(HL)2(phen)]), a 1D helical MOF (AEPF-16,
[Mg(H2O)2(L)(phen)]), and a 2D MOF (AEPF-17, [Mg-
(H2O)(L)(phen)]). The role that the synthesis conditions play
in the formation of each phase has been extensively studied.
Moreover, a combination of topological comparative analyses
Besides the choice of the metal ion, the type of ligand
(geometry, number/relative position of functional groups,
flexibility) is crucial in the formation of new nets. Moreover,
the choice of flexible ligands can induce some additional
interesting structural features, like reversible phase transitions2b
Received: November 10, 2011
Published: February 21, 2012
© 2012 American Chemical Society
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carried out by periodic Density Functional Theory (DFT). The DFT
plane-wave calculations were realized by using the VASP package.15,16 The
total energy was computed by using the revised Perdew-Burke-Ernzerhof
GGA17,18 exchange-correlation function. The effect of the core electrons on
the valence electron density was described by the projector augmented
wave (PAW) method.19,20 The cutoff for the kinetic energy of the plane-
waves has been set to 415 eV throughout, which after extensive test proved
to ensure a total energy convergence better than 10−5 eV. Geometry
optimization on selected starting geometries obtained from single crystal
X-ray diffraction was carried out by using a gradient-conjugate method. The
apparent formation energy was calculated as an energy difference between
the corresponding reagents and MOFs products.
with computational studies has been performed to determine
the relative energies for the five obtained networks.
The aim of this work is to emphasize the high significance of
fine-tuning the synthetic procedure to obtain each magnesium
compound a separated phase under hydro- or solvothermal
conditions, and to theoretically study the relative energies for
those compounds that coexist under certain hydrothermal
conditions, knowing in this way the thermodynamic or kinetic
control that drives each reaction.
EXPERIMENTAL SECTION
■
General Information. All reagents were purchased at high purity
(AR grade) and used without further purification. IR spectra were
recorded from KBr pellets in the range 4000−400 cm−1 on a Nicolet
FT-IR 20SXC spectrometer (see the Supporting Information, Section S4).
Thermogravimetric thermal analyses (TGA) were performed with a
SEIKO model EXSTAR 6300 apparatus in the temperature range
between 25 and 850 °C (Ar flow rate 50 mL·min−1) (see the Supporting
Information, Section S5).
RESULTS AND DISCUSSION
■
Effect of Synthesis Conditions. In this work five new
magnesium compounds based on the flexible H2L dicarboxylate
linker and the ancillary chelating N-donor phen ligand are going to
be discussed from the structural, energetic stability, and topological
point of view. The high number of obtained compounds proves
that a wide variety of supramolecular frameworks can be obtained
starting from the same primary building units (that is, octahedral
Mg2+ ions as metallic centers and H2L and phen as ligands). The
aim of this work is to emphasize the high influence of the synthetic
procedure terms on the formation of each of these magnesium
compounds under hydro- or solvothermal conditions. The
experiments performed during this investigation not only have
led to isolation of the highest number of compounds, but also to
understanding the influence of the experimental variables involved
in the formation of each magnesium framework.
Synthesis Procedures. The five compounds presented in this
work (named α-AEPF-14, β-AEPF-14, AEPF-15, AEPF-16, and
AEPF-17) were synthesized under hydro- or solvothermal conditions.
After optimizing the synthetic procedures, it was possible to obtain all
these compounds (except for AEPF-15) as pure crystalline phases. For
the best reaction conditions determined in each case see the
Supporting Information (Section S1).
Structure Determination by Single Crystal X-ray Diffraction.
Suitable crystals of α-AEPF-14, β-AEPF-14, AEPF-15, AEPF-16, and
AEPF-17 were selected under a polarizing optical microscope and were
glued on a glass fiber. Data were collected at 298 K on a Bruker four
circle κ-diffractometer equipped with a Cu microsource operated at 30 W
power (45 kV, 0.60 mA) to generate Cu Kα radiation (λ = 1.54178 Å),
and a Bruker AXIOM area detector (microgap technology) (β-AEPF-14
and AEPF-16) or an Bruker APEXII area detector (α-AEPF-14,
AEPF-15, and AEPF-17); exploring over a hemisphere of the
reciprocal space in a combination of φ and ω scans, using a Bruker
APEX2 software suite. Unit cell dimensions were determined by a
least-squares fit of reflections with I > 2σ(I). Data were then integrated
and scaled by using the SAINTplus6 program. Semiempirical
absorption and scale corrections based on equivalent reflections
were carried out with SADABS.7 Space group determinations and tests
for merohedral twining were carried out with XPREP.8 The structures
were solved by using the Direct Methods program SHELXS.8 The final
cycles of refinement were carried out by full-matrix least-squares
analyses with anisotropic thermal parameters for all non-hydrogen
atoms. All hydrogen atoms were placed in geometrically calculated
positions and subsequently refined by using a riding model with
Uiso(H) = 1.2Ueq(C), except for those of water molecules, which were
located from difference Fourier maps. The final structures were
examined and tested by using PLATON.9 CCDC reference numbers
CCDC 847926−847930 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Center.
Powder X-ray Diffraction. Powder X-ray Diffraction (PXRD)
measurements were performed with a Bruker D8 diffractometer in the
θ−θ mode, using nickel-filtered Cu Kα1,2 (λ = 0.15418 nm) radiation.
The best counting statistics were achieved by using a scanning step of
0.02° between 5° and 35° Bragg angles with an exposure time of 0.5 s
per step. Pawley refinements10 were performed by using Materials
Studio software11 for those compounds presented in this work that
have been obtained as pure phases.
Topological Analyses. The topological analyses of the AEPFs
presented in this work were performed with TOPOS software12
following the principle of underlying nets.13,14 In all cases, hydrogen
bond interactions were taken into account to study the topology of
supramolecular frameworks.
The hydrothermal reaction of magnesium acetate tetrahy-
drate and H2L with phen as chelating ligand at 160 °C during
2 days gave rise to a heterogeneous crystalline product (the
composition of the reaction mixture was 1 Mg2+:1 phen:1 H2L
molar ratio). After careful analysis of this product, single
crystals with different morphologies were identified and their
crystal structures were solved by single crystal X-ray diffraction
experiments, which corresponded to different novel magnesium
compounds: α-AEPF-14 ([Mg(H2O)4(phen)]L), AEPF-15
([Mg(HL)2(phen)]), and AEPF-16 ([Mg(H2O)2(L)(phen)]).
With the aim of obtaining these three compounds as pure
crystalline phases, a rational optimization of the hydrothermal
synthesis conditions (SC), was carried out (Table 1). The effect
Table 1. Explored Hydrothermal Synthesis Conditions on
the System 1 Mg+2:1 phen:1 H2L
SC
T (°C)
time
1 h
pH
phase
1
160
160
160
160
170
170
170
170
180
180
180
180
180
180
200
200
200
6
6
6
6
6
6
6
6
6
6
6
7
7
7
6
6
6
α-AEPF-14
2
2 h
α-AEPF-14 > AEPF-15, AEPF-16
α-AEPF-14 > AEPF-15, AEPF-16
AEPF-15 > α-AEPF-14, AEPF-16
α-AEPF-14, AEPF-15
AEPF-15 ≫ AEPF-16
AEPF-15 ≫ AEPF-16
AEPF-15 ≫ AEPF-16
AEPF-16, AEPF-17
3
3 h
4
2 days
1 h
5
6
2 h
7
3 h
8
2 days
1day
9
10
11
12
13
14
15
16
17
5 days
10 days
1 days
5 days
10 days
1 day
5 days
10 days
AEPF-17 > AEPF-16
AEPF-17 > AEPF-16
AEPF-16 > AEPF-17
AEPF-16
AEPF-16
AEPF-16 > AEPF-17
AEPF-16, AEPF-17
Computational Details. The structural stability ab initio
calculations of the five compounds presented in this work have been
AEPF-17 > AEPF-16
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of the reaction temperature and time (without changes in the
composition of the reaction mixture) was systematically studied
and the main results are discussed in the following.
observed that the heating rate played an important role. Up to
the moment, all the experiments were performed placing the
In a first step, kinetic parameters (without temperature changes)
were evaluated (Table 1, SC1−SC4), showing the formation of α-
AEPF-14 compound as a pure phase when short reaction times
are used (1 h) (SC1). The increase of reaction time up to 2 and
then to 3 h (SC2 and SC3, respectively) gave rise to a mixture of
α-AEPF-14, AEPF-15, and AEPF-16 in which α-AEPF-14 was
the principal component. The proportion of this mixture changed
after 2 days of reaction, when AEPF-15 compound became the
principal component of the mixture (SC4).
In order to study the thermodynamic effects, a series of
reactions were also performed increasing the reaction temperature
up to 170 °C (Table 1, SC5−SC8). Thus, when short reaction
times are used (1 h), a mixture of α-AEPF-14 and AEPF-15 was
determined (SC5) (∼50% of each phase). An increase of the
reaction time up to 2 and then to 3 h (SC6 and SC7) led to a
crystalline mixture for which the main component was AEPF-15,
together with traces of AEPF-16. The same result was obtained
after increasing the time of reaction up to 2 days (SC8).
Taking into account the important role that both the
thermodynamic and the kinetic effects play, several experiments
were then performed at a higher temperature (180 °C) and
longer reaction times (1, 5, and 10 days) (Table 1, SC9−
SC11). Under these hydrothermal conditions, when the time of
reaction was 1 day, a mixture of AEPF-16 and a new phase
named AEPF-17 was obtained (∼50% of each) (SC9).
Increasing the reaction time up to 5 and 10 days (SC10 and
SC11), the contribution of AEPF-17 in the mixture increased.
However, it was not possible to obtain either of the two
compounds as pure phases.
At this point, the pH was also considered as an additional
variable. Thus, in the next step, the effect of the pH increasing
up to 7 by using a NaOH solution (0.1 M) was investigated
(Table 1, SC12−SC14). By comparison of the sample obtained
after 1 day of reaction at pH 7 (SC12) to that obtained at pH 6
(SC9), it could be concluded that a higher pH value favors the
formation of AEPF-16 instead of AEPF-17. Finally, at longer
time periods (5 and 10 days) (SC13-SC14), AEPF-16 was
successfully isolated.
Once the AEPF-16 compound was synthesized as a pure
phase, several experiments were carried out to establish the
reaction conditions under which AEPF-17 could be isolated.
With this propose, a series of hydrothermal reactions were
performed to evaluate the effect of a higher temperature (200 °C)
(Table 1, SC15−SC17). Thus, after 1 day of reaction, a mixture
of AEPF-16 (major component) and AEPF-17 was obtained
(SC15). When longer reaction times are used, AEPF-17
became the principal mixture component (SC16−SC17).
However, yet under these conditions it was not possible to
isolate AEPF-17.
Table 2. Explored Solvothermal Synthesis Conditions on the
System 1 Mg+2:1 phen:1 H2L
SC
T
(°C)
time
(days)
H2O:Me2CO
phase
heating
16
18
200
200
5
5
1:0
1:0
AEPF-16, AEPF-17
fast
β-AEPF-14,
amorphous
slow
19
20
21
22
23
24
200
200
200
200
200
200
5
5
5
5
5
5
20:1
5:1
2:1
1:1
1:5
0:1
AEPF-17
amorphous
β-AEPF-14
β-AEPF-14
liquid
slow
slow
slow
slow
slow
slow
liquid
autoclave reactors in a preheated oven at the desired
temperatures (fast heating, SC16). However, in this case,
certain hydrothermal conditions were chosen and the reaction
was then repeated by using a slow heating treatment (5 °C·min−1,
slow heating, SC18). Surprisingly, under this slow heating
condition, a new compound was determined (named β-AEPF-
14, which is a polymorph of α-AEPF-14), together with an
important amorphous contribution. Taking into account this
interesting result, an optimization of solvothermal conditions
was carried out by using slow heating treatments (T = 200 °C
and 5 days reaction time), systematically varying the polarity of
the reaction media (H2O:Me2CO volume ratio). The main
results are presented in Table 2.
Thus, a decrease in the polarity (H2O:Me2CO volume ratios =
2:1 and 1:1) led to the successful formation of β-AEPF-14
as a pure phase (SC21 and SC22). In addition, at higher
H2O:Me2CO volume ratio of 20:1, AEPF-17 was also
synthesized as a pure phase (SC19). Finally, when the mixture
is mainly or totally composed by Me2CO (SC23 and SC24,),
the solvothermal reactions resulted in the formation of liquids.
To summarize all the above-mentioned results, after
rationally optimizing the synthesis procedures: (i) Two
AEPF-14 polymorphs were obtained as pure crystalline phases
under hydrothermal conditions (α-phase, SC1) and solvother-
mal conditions (β-phase, SC21) varying the polarity media; (ii)
In addition, AEPF-16 compound was isolated by increasing the
pH (SC13). (iii) AEPF-17 (as it occurs for β-AEPF-14) is
obtained as a pure crystalline phase under solvothermal
conditions, using slow heating treatments (SC19). Schemes 1
and 2 summarize the synthesis optimization procedures carried
out under hydrothermal and solvothermal conditions, respec-
tively, to obtain each new magnesium phase.
Crystal Structure Description and Topological Ana-
lyses. Details of the single crystal X-ray diffraction data
collection, cell parameters, and crystal structure refinement for the
five compounds presented in this work are given in Tables S1−S5
(Supporting Information). The corresponding ORTEP represen-
tations for these crystal structures are shown in Figures S1−S5
(Supporting Information). A summary of the structural and
topological features for AEPF-14 (α- and β-), AEPF-15, AEPF-
16, and AEPF-17 are depicted in Figure 1. Details about the
topological simplifications carried out are shown in Figure 2.
AEPF-14 Polymorphs. The [Mg(H2O)4(phen)]L com-
pound (AEPF-14) presents two different polymorphs, named
α- and β-phases, which were isolated under hydro- and
Taking into account the unsuccessful efforts to purify the
AEPF-17 phase under high-temperature hydrothermal con-
ditions (200 °C), we then introduced changes in the polarity of
the reaction media. For that purpose, a mixture of acetone
(Me2CO) and water was chosen as solvent media, using
different volume ratios (Table 2). It is worth mentioning that,
although a wide variety of conditions were investigated during
this work (the studied variables were as follows: time,
temperature, and H2O:Me2CO volume ratio), at the beginning
all the experiments resulted in the formation of liquids.
Moreover, during the development of this investigation it was
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Scheme 1. Summary of the Explored Hydrothermal Synthesis Conditions
Scheme 2. Summary of the Studied Solvothermal Synthesis Conditions
Figure 1. Summary of the structural and topological features of compounds in which inorganic chains are formed (a) via hydrogen bonds (AEPF-14
(α- and β-phases) and AEPF-16) and (b) via covalent bonds (AEPF-15 and AEPF-17). Hydrogen bonds determined in these materials are depicted
in red.
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Table 3. Summary of the Structure Parameters Determined for AEPF-14 Polymorphs
AEPF-14
α-phase
β-phase
empirical formula
formula weight
wavelength
C29 H24 F6 Mg N2 O8
666.81
1.54178 Å
crystal system
space group
monoclinic
orthorhombic
P21/c
Pnna
unit cell dimensions
a = 13.2022(2) Å
b = 7.6135(1) Å
c = 31.0128(5) Å
3071.18(8) Å3
4
a = 7.3064(2) Å
b = 32.602(1) Å
c = 12.5442(4) Å
2988.11(16) Å3
4
β = 99.863(1)°
volume
Z
independent reflecns
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
4821 [R(int) = 0.0296]
4821/8/447
2434 [R(int) = 0.0726]
2434/4/226
1.055
1.035
R1 = 0.0464, wR2 = 0.1191
R1 = 0.0650, wR2 = 0.1322
R1 = 0.0460, wR2 = 0.1261
R1 = 0.0548, wR2 = 0.1329
Figure 2. Summary of the topological features of compounds AEPF-14 (α- and β-phases), AEPF-15, AEPF-16, and AEPF-17. (a) Simplification
points performed to describe the inorganic SBUs and (b) simplified inorganic SBUs. (c) Simplification points performed to describe the nets and (d)
simplified nets for the five compounds.
solvothermal conditions, respectively. The monoclinic α-AEPF-
14 polymorph crystallizes in the P21/c space group, and the
orthorhombic β-AEPF-14 crystallizes in the Pnna space group.
A summary of the structure parameters for both polymorphs is
shown in Table 3.
inorganic chains parallel to the b and a axes for α- and β-
polymorphs, respectively (Figure 1a). In both cases the in-
organic chains are hydrogen linked to each other via com-
plete L2− linker. Nevertheless, while in α-AEPF-14 each
chain is joint to the other two in a 2D way, in β-polymorph a
3D net comes up, as a consequence of a higher number of
connections (four in this case) among chains (Figure 1b).
The β-AEPF-14 depicted features, together with the pre-
sence of π−π stacking interaction among the L2− aromatic
rings (distances among centroids 3.655 Å), create a more
compact structure, in which there is not any accessible free
space. In α-polymorph, with a 2D supramolecular net and
absence of π−π interaction, a 4.6% free space (140.4 Å3 per
unit cell) was determined by PLATON (cavity routine).9
To consider topological features of AEPF-14 polymorphs,
the simplifications of their supramolecular networks were
performed, taking into account an interesting work of Yaghi
and O’Keeffe,21 in which the authors carefully studied the rod-
packing phenomenon in MOFs. Thus, by considering the
infinite (−Mg−O···O−C−O···O−Mg−)∞ rods found in both
AEPF-14 polymorphs as the inorganic secondary building units
(SBUs) (the carboxylate was simplified as O−C−O), the two
nets then can be simplified in an elegant way (Figure 2a,b).
In both AEPF-14 polymorphs, the Mg2+ ion is hexa-
coordinated to four oxygen atoms of water molecules and two
nitrogen atoms coming from one phen ligand, to form distorted
MgN2O4 octahedra. These inorganic polyhedra, which can be
considered as inorganic primary building units (PBUs), do not
comprise thus, oxygen atoms of the L2− carboxylate groups. In
both cases, hydrogen bonds between coordinated water
molecules and L2− carboxylate groups govern the supra-
molecular interactions. However, while in α-AEPF-14 eight
different kinds of strong hydrogen bonds were determined, in
β-AEPF-14 there are only three of them. To clarify this point,
which is crucial to understand the supramolecular features that
make the difference between the two polymorphs, hydrogen
bond distances and angles for both AEPF-14 compounds were
analyzed in detail and presented in Table S6 (Supporting
Information). When taking into account these interactions,
MgN2O4 polyhedra are hydrogen bridged via carboxylate
oxygen atoms, giving rise to (−Mg−O···O−C−O···O−Mg−)∞
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Figure 3. (a) Depiction along the b axis of the very decorated SBUs built as an extension of the inorganic chains. (b) Two AEPF-15 decorated SBUs
hydrogen bounded via the linker carboxylic groups.
Figure 4. AEPF-16 supramolecular framework built up via hydrogen bonds. (a) Double [M−L]∞ helical covalent chain, showing different
orientations. (b) Junction of two double [M−L]∞ helical chains along the c axis.
Taking into account these topological considerations, these
SBUs can be described as “ladders” formed by sharing opposite
edges of quadrangles. Regarding the α-AEPF-14 supra-
molecular framework, these SBUs are linked with the rungs
in parallel “parallel rung ladders” to form a uninodal four-
connected 2D sql net (point symbol (44.62)) (the nodes are
located on C atoms of the linker carboxylate groups). However,
in the case of the β-AEPF-14 compound, as was mentioned
before, each rod is linked to four others, giving rise to a three-
dimensional net. Thus, the parallel connection of the rungs in
this compound is made in a different way giving rise to a
uninodal four-connected 3D sra net, whose nodes also
correspond to C atoms of the linker carboxylate groups. The
main simplification points, as well as the final simplified nets for
both AEPF-14 polymorphs, are shown in Figure 2c,d.
AEPF-15. The hydrothermal reaction between H2L and
magnesium acetate tetrahydrate, using phen as chelating ligand,
has also given rise to the [Mg(HL)2(phen)] compound
(AEPF-15), in which the organic linker remains half
protonated as the HL− anion. AEPF-15 crystallizes in the
monoclinic crystal system (C2/c space group) (see Table S6,
Supporting Information, for all parameters).
In AEPF-15 the MgN2O4 octahedral PBU is formed by
Mg2+ coordination to four oxygen atoms coming from HL−
carboxylate groups and the two nitrogen atoms of the chelat-
ing phen molecule. Linkages of the later form inorganic
chains (SBUs) along the c axis, through the carboxylate group
that acts in a η2μ2 coordination mode (Figure 1b). As in this
compound the linkers are only covalently bonded to the Mg
ions by the deprotonated carboxylate groups, they hang from
the PBUs, giving rise to very decorated SBUs (Figure 3a).
Typical hydrogen bonds between HL− carboxylic acids
(Figure 3b and in the Supporting Information Table S7) give
rise to the formation of a three-dimensional supramolecular net
(Figure 1b).
Concerning the topology of the supramolecular net
determined for AEPF-15, the inorganic PBUs are connected to
each other through the η2μ2 coordinated carboxylate groups,
giving rise to (−Mg−O−C−O−Mg−)∞ rods21 considered as
SBUs. By simplification of these SBUs, one type of four-connected
node comes up, which is situated in the carboxylate group C
atoms (Figure 2a,b). Thus, the SBU appears as in the former
compounds as ladders, but in this case, and probably due to the
fact that the linker is directly coordinated to the metal in a bridging
way, the rungs are at an angle to each other, so we call them
“twisted ladders” (Figure 1b). Through linkages among them via
hydrogen bond interactions, a 3D irl supramolecular net is built
(point symbol (42.63.8)) (Figure 2c,d).
AEPF-16. As it was mentioned above in the Synthesis
Procedures section, when the pH of the reaction mixture is
slightly increased under certain hydrothermal conditions, the
novel compound [Mg(H2O)2(L)(phen)] (AEPF-16) is iso-
lated as pure phase. This material crystallizes in the
orthorhombic crystal system (Pnna space group) (Table S6,
Supporting Information). In AEPF-16, Mg2+ ions are
coordinated to two oxygen atoms coming from L2− carboxylate
groups, two oxygen atoms of water molecules and the two phen
nitrogen atoms giving the MgN2O4 octahedral PBU. These
PBUs are covalently bonded through the deprotonated L2− ligand,
which acts in a η1−η1 coordination mode, leading to the formation
of 1-D polymeric helical chains [M−L]∞ (Figure 4a). The
supramolecular interactions in this compound are governed by
three different hydrogen bonds, which are determined between
coordination water molecules and L2− carboxylate groups. The
distances and angles of these interactions are listed in Table S8,
Supporting Information. If these hydrogen bonds are taken into
account, the MgN2O4 PBUs are additionally hydrogen bridged via
L2− carboxylate groups, giving rise to (−Mg−O···O−C−O···O−
Mg−)∞ inorganic chains (Figure 1a). The AEPF-16 supra-
molecular net consists of double helical [M−L]∞ chains, parallel
to the b axis, joint among them, via hydrogen bonds in the
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Figure 5. (a) Polyhedral view of two layers in AEPF-17 layers viewed along the b axis and detail of the inorganic chains, showing the hydrogen
bonds in red. (b) Depiction of two undulated layers in AEPF-17 viewed along the a axis.
c direction (Figure 4). The result is a three-dimensional
supramolecular framework (Figure 1a).
Structural Stability Studies: Theoretical Calculations.
The use of theoretical calculations as a complement in the
determination of the factors that govern the mechanisms of
MOFs formation processes has been proved to be very
useful.22−26 In order to determine the structural stability of the
magnesium frameworks presented in this work and to deepen
the understanding of their formation pathways, a series of
theoretical DFT plane-wave based calculations were carried out.
The apparent formation energies for the studied compounds
were obtained by using the VASP package.15,16 In all cases, the
geometry optimization converged to a stable structure with the
same topology as that determined experimentally, even though
no symmetry constraints were imposed.
To consider topological features of AEPF-16, the (−Mg−
O···O−C−O···O−Mg−)∞ inorganic chains were described as
rods21 (SBUs). By their simplification, only one type of four-
connected node appears, which corresponds to the C atoms of
carboxylate groups (Figure 2a,b). Then, as occurs in AEPF-14
polymorphs, the SBU consists of rods sharing opposite edge
quadrangles named “parallel rung ladders” (Figure 2b). Covalent
joints of these ladders through the whole L2− linker give rise to a
3D sra net. The main simplification points as well as the final
simplified net for AEPF-16 are shown in Figure 2c,d.
AEPF-17. Finally, under solvothermal conditions, a novel
compound with formula [Mg(H2O)(L)(phen)] (AEPF-17) is
obtained as a pure phase. AEPF-17 crystallizes in the
orthorhombic crystal system (Pbca space group) (Table S6,
Supporting Information). The Mg2+ ions are bonded to three
oxygen atoms coming from L2− carboxylate groups, one water
molecule and two phen nitrogen atoms. These PBUs are
connected to each other through the η2μ2 coordinated
carboxylate groups, giving rise to (−Mg−O−C−O−Mg−)∞
inorganic chains parallel to the b axis (Figure 1b). Additional
intrachain hydrogen bonds are found, due to the presence of
coordinated water molecules and carboxylate groups that act in
a η1 mode. The distances and angles of these hydrogen bonds
are listed in Table S9, Supporting Information. It is worth
mentioning that the presence of π−π stacking interactions is
found among phen rings, with a distance among centroids of
3.743 Å.
On one hand, the polymorphism phenomenon found for the
AEPF-14 compound was analyzed in detail by using computa-
tional studies. With that purpose, the relative energies for the
two AEPF-14 phases (α- and β-) were determined by DFT-
based calculations. As is shown in Figure 6, after comparing the
In AEPF-17, each inorganic chain described above is bonded
to two others via the L2− linker, giving rise to undulated layers
perpendicular to the a axis (Figure 5). The described hydrogen
bonds do not increase the net dimensionality since they are
between the intra chains.
Figure 6. Relative energies per volume for both AEPF-14 polymorphs
(α- and β-phases).
Concerning the topology of AEPF-17 net, as was done
before, the inorganic SBUs (−Mg−O−C−O−Mg−)∞ were
simplified as rods,21 giving rise to one type of four-connected
nodes, situated in the carboxylate group C atoms (Figure 2a,b).
In doing that, the SBU appears also as “twisted ladders” (Figure 1b).
Covalent bonding among them via the linker results in a 2D sql
net (point symbol (44.62)) (Figure 2c,d).
X-ray Powder Diffraction Studies. X-ray diffraction data
were analyzed by Pawley10 refinements methodology, using
Materials Studio software.11 For those compounds that have
been isolated after optimizing the synthesis procedures, these
structural studies have shown the presence of a unique
crystalline phase, demonstrating the purity of the bulk samples.
Pawley refinement profiles and the main refinement values for
AEPF-14 (α- and β-), AEPF-16, and AEPF-17 are shown in
Figures S6−S9 and Tables S11−S14 in the Supporting
Information.
relative energies per volume for both structures, it can be
concluded that the β-phase is the most stable one. This fact is
in good agreement with the higher dimensionality of the
supramolecular net of this polymorph (3D) and with the
obtained results in the synthesis procedure.
In addition, in order to correlate the structural stability with
the synthesis procedures, a series of computational studies were
performed to determine the formation energies (EFormation) for
those compounds that coexist under certain conditions (Figure 7).
Regarding the hydrothermal reactions performed at lower
temperatures (T = 160−170 °C) and short reaction times (1 h
to 2 days) (Figure 7A), the coexistence of α-AEPF-14, AEPF-
15, and AEPF-16 compounds has been observed. In addition,
the ratio among the phases varies through the time. Thus, while
at 160 °C α-AEPF-14 is the principal component, when shorter
reaction times are used, AEPF-15 and AEPF-16 become the
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obtained EFormation values for these compounds (Figure 7), it
can be suggested that a competition between both thermody-
namic and kinetic processes takes place under these conditions.
However, when longer reaction times are used, the more stable
AEPF-17 compound is mainly obtained (thermodynamic
control). At 200 °C, an increase of the kinetically controlled
phase (AEPF-16) is observed.
Dehydration Processes: AEPF-14 Polymorphs and
AEPF-16. As was mentioned above, both β-AEPF-14 and
AEPF-16 compounds exhibit 3D supramolecular nets that
belong to the sra topological type. On the other hand, they
present the same Mg:phen:L ratio per molecular formula, but
different numbers of coordinated water molecules per metal
ion. In this context, we were interested in studying the
dehydration processes of these compounds, in order to
elucidate some common structural changes during the loss of
their coordinated water molecules. This study was also
extended to the α-AEPF-14 compound, to determine the
possible structural transformations between both AEPF-14
polymorphic forms.
Taking into account TGA results (see the Supporting
Information, Section S4), both AEPF-14 samples were heated
under vacuum to ensure the removal of coordination water
molecules (α-AEPF-14 at 150 °C and β-AEPF-14 at 180 °C).
The dry samples were examined by PXRD in order to
determine the structural changes provoked by the dehydration.
Although for both polymorphs an important loss of crystallinity
was detected after this heating treatment, the indexation of their
PXRD patterns could be performed by using DICVOL0427,28
software, implemented in the FullProf package.29 In addition, in
order to compare the obtained results, the same study was
performed with AEPF-16 compound (the sample was heated at
150 °C). The cell parameters found for the dry samples,
together with those determined by single crystal X-ray
diffraction for the as-synthesized compounds, are shown in
Table 4.
Taking into account these results, several conclusions can be
drawn. Regarding the behavior of AEPF-16 during the
dehydration process, the compound suffers an isotropic
contraction (∼1.5%) of its cell parameters, giving rise to a
reduction of 4.3% in its cell volume (VAEPF‑16 = 2808.1 Å3,
VdryAEPF‑16 = 2686.8 Å3) (Table 4), retaining the orthorhombic
crystal system and its crystallinity.
Figure 7. (Top) Calculated formation energies (ΔEFormation) for α-
AEPF-14, AEPF-15, AEPF-16, and AEPF-17 compounds. (Bottom)
Scheme of the experimentally determined phases’ formation under soft
reaction conditions (A) and hard reaction conditions (B).
principal components of the products mixture for longer
reaction times. Taking into account the results coming from
formation energies (Figure 7), several conclusions can be
drawn. First, α-AEPF-14 presents the lowest EFormation among
these three phases, which seems to indicate that, at shorter
reaction times, thermodynamic processes govern the reaction.
However, with the increase in reaction time and temperature
(170 °C), the reaction obeys a kinetic control, which leads to the
formation of the less stable phases (AEPF-15 and AEPF-16).
Concerning the hydrothermal reactions performed at higher
temperatures (T = 180 and 200 °C) and long reaction times
(1−10 days) (Figure 7B), the coexistence of AEPF-16 and
AEPF-17 compounds has been observed. Moreover, the ratio
among the phases varies through the time. Thus, at 180 °C,
after 1 day of reaction, AEPF-16 and AEPF-17 are found in a
similar proportion; with the increase of the reaction temper-
ature, AEPF-16 becomes the major component of the products
mixture. In addition, the use of longer reaction times leads to
the formation of AEPF-17 as a principle phase. Taking into the
In the case of AEPF-14, both polymorphs exhibit different
structural transformations when losing their coordinated water
molecules, probably due to different topologies found in their
supramolecular nets. Thus, while β-polymorph exhibits double
helical chains joint along the a and the c axes via hydrogen
bonds (sra net), for α-polymorph only undulating double
chains joint along the b axis can be found (sql layers). As is
Table 4. A Summary the Cell Parameters for AEPF-14 (α- and β-) and AEPF-16 As-Synthesised (Found by Single Crystal X-ray
Diffraction) and after the Dehydration Process (Found by PXRD)
α-AEPF-14
dry α-AEPF-14
β-AEPF-14
dry β-AEPF-14
AEPF-16
dry AEPF-16
9.1736(8)
́
́
a/Å
13.2022(2)
7.6135(1)
31.0128(5)
90.0
11.00(11)
9.11(2)
7.3064(2)
32.6024(10)
12.5442(4)
90.0
6.406(8)
27.404(24)
11.745(12)
90.0
9.3105(3)
26.8940(8)
11.2146(3)
90.0
b/Å
26.488(5)
11.057(3)
90.0
́
c/Å
27.52(4)
90.0
α/deg
β/deg
99.863(1)
90.0
97.6(7)
90.0
90.0
90.0
90.0
γ/deg
V/Å3
90.0
90.0
90.0
90.0
90.0
3071.18(8)
2735.25
2988.11(16)
2062.01
2808.10(14)
2686.82
M20 = 35, F20 = 68
figures of merit
M12 = 12, F12 = 19
M14 = 10, F14 = 12
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Figure 8. Structural study performed with dehydrated samples for (a) α-AEPF-14, (b) β-AEPF-14, and (c) AEPF-16. On the left, comparisons of
PXRD patterns before and after the dehydration treatment are shown. On the right, stick and space filling representations of the double chain
structures are depicted.
shown in Table 4, after dehydration the orthorhombic β-
polymorph retains its crystal symmetry although it suffers an
important contraction of the b axis (∼16%) (along which the 21
axis runs) and the a axis (∼12%) (along which the π−π
stacking runs). These results point out the first conclusion: the
similarities found between the cell parameters of dry β-AEPF-
14 and dry AEPF-16 allow us to elucidate a transformation of
both compounds to a similar orthorhombic phase with a double
helical chain structure (Figure 8b,c). Considering the
topological similarities between the two compounds, the
structural transformation caused by the loss of coordinated
water molecules can be explained as a subtle contraction of this
1D helical structure.
In the case of the dry α-AEPF-14 compound, its PXRD
pattern was indexed to a monoclinic cell, which exhibits similar
cell parameter values to those of dry AEPF-16 but a value of β
of ∼98°, which is far from the orthorhombic crystal system
(Table 4). So, the second conclusion that can be drawn is the
following: starting from α-monoclinic polymorph, in which
the double chains are not joint via interchain hydrogen bonds,
the formation of the 1D helical structure does not take place
(Figure 8a). Thus, the structural transformation that suffers α-
AEPF-14 after the dehydration treatment can be caused by
little changes in the undulating packing of these double layers.
and the relative formation energies for the compounds that coexist
under certain hydrothermal conditions, the thermodynamic or
kinetic control of the reaction have been established. α-AEPF-14
can be regarded as the precursor, in which Mg2+ ions are
coordinated, besides to the blocking ligand, only to water molecule
oxygen atoms. When reaction time goes on, it gets dissolved, and
then the less stable phases, AEPF-15 and AEPF-16, whose Mg
coordination spheres comprise carboxylate oxygen atoms, energeti-
cally compete to come up in a kinetically controlled process. The
energetically more stable AEPF-17 appears at higher temperature
and much longer reaction time, the process being thus
thermodynamically controlled.
Considering structural and topological features, hydrogen bonds
govern the supramolecular interactions in the five compounds,
achieving uninodal four-connected supramolecular nets with
higher dimensionality in all the cases except for AEPF-17,
where the chelating ligands hinder any interlayer hydrogen bond
interaction. In those cases in which O−C−O covalent bridges join
the inorganic polyhedra to form the SBU in the rod packing
simplification, the later results in a “twisted ladder”, while in those
compounds in which the linkage among polyhedra is made only
via hydrogen bonds the SBU is a regular “parallel rung ladder”.
In addition, the AEPF-14 polymorphism was studied in
detail, determining that β-polymorph being the most stable,
only can be obtained via solvothermal synthesis in a media in
which one-third of water was substituted by acetone in volume.
Finally, the dehydration studies performed on AEPF-14 (α-
and β-) and AEPF-16 have shown that the topology of their
supramolecular networks determines the structural changes that
take place during the dehydration processes of these Mg
compounds.
CONCLUSIONS
■
In summary, the effect of introducing of N-donor chelating
ancillary ligands on the synthesis of Mg MOFs has been explored,
resulting in five new compounds, which exhibit 0D, 1D, and 2D
dimensionalities, and 2D and 3D supramolecular frameworks: a
molecular magnesium material with polymorphism (named
AEPF-14, α- and β-phases), a 1D MOF (AEPF-15), a 1D
helical MOF (AEPF-16), and a 2D MOF (AEPF-17). The role
that the synthesis conditions play in the formation of each phase
has been exhaustively studied and complemented with theoretical
calculations. Taking into account both the results of the synthesis
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthetic procedures, structural analyses by single crystal X-ray
diffraction, powder X-ray diffraction, infrared spectroscopy, and
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(22) Lee, C.; Mellot-Draznieks, C.; Slater, B.; Wu, G.; Harrison,
W. T. A.; Rao, C. N. R.; Cheetham, A. K. Chem. Commun. 2006, 25,
2687.
thermal gravimetric analyses, along with ORTEP representa-
tions for these crystal structures shown in Figures S1−S5. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(23) Bernini, M. C.; de la Pena-O’Shea, V. A.; Iglesias, M.; Snejko,
̃
N.; Gutierrez-Puebla, E.; Brusau, E. V.; Narda, G. E.; Illas, F.; Monge,
́
M. A. Inorg. Chem. 2010, 49, 5063.
AUTHOR INFORMATION
(24) Keskin, S.; Liu, J.; Rankin, R. V.; Karl Jhonson, J.; Sholl, D. S.
Ind. Eng. Chem. Res. 2009, 48, 2355 and references therein.
(25) Coombes, D. S.; Cora, F.; Mellot-Draznieks, C.; Bell, R. G.
J. Phys. Chem. C 2009, 113, 544.
■
Corresponding Author
*amonge@icmm.csic.es
(26) Sillar, K.; Hofmann, A.; Sauer, J. J. Am. Chem. Soc. 2009, 131
(11), 4143.
Notes
The authors declare no competing financial interest.
(27) Louer, D.; Louer, M. J. Appl. Crystallogr. 1972, 5, 271.
̈
̈
(28) Boultif, A.; Louer, D. J. Appl. Crystallogr. 1991, 24, 987.
̈
(29) Rodriguez-Carvajal, J. Physica B 1993, 192, 55.
ACKNOWLEDGMENTS
■
This work has been supported by the Spanish MICINN Projects
MAT2010-17571, MAT2009-09960, CTQ 2007-28909-E/BQU,
CAM: S2009/MAT-1756/CAM, and Consolider-Ingenio
CSD2006-2001. A.E.P.P. acknowledges a JAE fellowship from
CSIC and Fondo Social Europeo from EU. V.A.P.O. acknowl-
edges financial support from the MCYT in the Ramon y Cajal
Program and ENE2009-09432. Computational time has been
provided by the Centre de Supercomputacio
(CESCA) and Centro de Supercomputacio
(CESGA) by generous grants.
́
de Catalunya
́
n de Galicia
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